Multicomponent Reactions , Solvent-Free Synthesis of 2-Amino-4-aryl-6-substituted Pyridine-3 , 5-dicarbonitrile Derivatives , and Corrosion Inhibitors Evaluation

A number of 2-amino-4-aryl-6-substituted pyridine-3,5-dicarbonitrile derivatives were synthesized via one-pot multicomponent condensation reactions of different aromatic aldehydes with malononitrile and different primary amines, using different molecular ratios and different reaction conditions to achieve considerable product yields. Moreover, we succeed, for the first time to develop a newmethod to synthesize the aforementioned under the fusion condition without using solvent and catalysts. With this method, a wide range of novel 2-amino-3,5-dicyano-4-aryl-6-substituted aminopyridine derivatives were synthesized with high yields and board substrate of functional groups. .e synthesized pyridine derivatives were found to have a corrosion inhibition efficiency, the rate of which increased with the increasing concentration of the derivatives. .e structures of the new compounds were elucidated by spectroscopic data and elemental analyses.


Introduction
Multicomponent reactions (MCRs) have drawn high efforts in recent years owing to exceptional synthetic efficiency, high selectivity, and procedural simplicity [1][2][3][4][5][6].One-pot multicomponent reactions are a new method to construct heterocyclic compounds with bond making and/or bond breaking in one step with high atom economy, and the diversity can be achieved by varying the reacting components.A large number of organic reactions which were carried out afforded higher yields, shorter reaction time, and milder conditions [7][8][9].
Substituted pyridines were used as medical scaffolds because they are part of many natural product structures [10,11].Pyridine derivatives have also showed a broad spectrum of biological activities such as antimitotic agents [12], anti-inflammatory substances [13], and anticonvulsants [14].In addition, they also regulate arterial pressure [15] and cholesterol level in blood [16].Furthermore, they were utilized as electrical materials [17] and chelating agents [18].Furthermore, organic compounds rich in heteroatoms behave as corrosion inhibitors, when they are absorbed in the metal surface to form a protective layer preventing cathodic and/or anodic reactions and forming a compact barrier film [19][20][21].
From the synthetic aspect, most of the existing studies on 2-amino-4-aryl-6-substituted pyridine-3,5-dicarbonitrile derivatives [22] were synthesized by ZnCl 2 -catalyzed multistep methods [23] and one-pot multicomponent reactions [24] with good yield.e addition of corrosion inhibitors is a useful approach to protect mild steel (MS) surfaces from corrosion damage [25].Considerable efforts are made to synthesize new organic molecules offering various molecular structures.e most synthesized compounds are the nitrogen-heterocyclic compounds which are known to be excellent complex or chelate-forming substances with metals of transition series [26].Also, the heterocyclic compounds containing nitrogen atoms can be easily protonated in the acidic medium to exhibit good inhibitory action [27].
In our preliminary studies, we have investigated the onepot multicomponent reactions of different aromatic aldehydes with malononitrile and primary amines, using various Lewis acids such as ZnCl 2 , AlCl 3 , and FeCl 3 in ethanol as a solvent with different molecular ratios.
e pyridine derivatives were obtained in good yield using solvent and catalyst-free condition under the fusion condition.Aminopyridine derivatives and their corrosion inhibition properties were evaluated by weight loss measurements of steel with different concentrations of inhibitors.
In our present study, we have investigated the one-pot multicomponent reactions (MCRs) to prepare aminopyridine derivatives 1-20 using different Lewis acid catalysts such as ZnCl 2 , AlCl 3 , and FeCl 3 using a molar ratio of 1 : 2 : 3 (aromatic aldehydes: malononitrile: primary amines) where the desired products were obtained in moderate yields.
e first trial was performed using one equivalent of aromatic aldehydes, two equivalents of malononitrile, and one equivalent of primary amines in the presence of ZnO (nano particles), CAN, NaOEt, and/or H 3 PO 4 as catalysts in refluxing ethanol for 12 h, and no products were observed even after changing the molar ratio of amines to two equivalents and/or three equivalents (Table 1, entries 1-12) (Scheme 1).
e formation of the desired products 1-20 could be explained via the formation of the arylidene molecule than the Michael addition of malononitrile on the double bond of the arylidene moiety forming the intermediate which underwent cyclization by the nucleophilic attack of amine on the cyanocarbon followed by aromatization to the final products 1-20 (Scheme 3).
As a conclusion, we have developed for the first time a solvent-free, one-pot multicomponent reaction without using any catalysts.With this method, a wide range of novel 2-amino-4-aryl-6-substituted pyridine-3,5-dicarbonitrile derivatives were synthesized in high yields with a board substrate of functional groups.
ose derivatives are depicted in Scheme 4.

Evaluation of the Synthesized 2-Amino-4-aryl-6substituted Pyridine-3,5-dicarbonitrile as Corrosion
Inhibitors.Alloys are exceedingly applied in manufacture processing applications and might undergo various acidic mediums.Acids were aggressive on the metal surface and progress to serious corrosion issues.Corrosion has been controlled by employing natural or synthetic inhibitors.Most of the utilizing inhibitors were from organic molecules with heteroatoms such as nitrogen, sulphur, phosphorus, and oxygen in addition to double bonds or aromatic rings in their structure that adsorbed on the surface of the metals.Corrosion inhibitors usually have the ability to control the corrosion through forming different kinds of films in various routes, such as adsorption through formation of precipitates or through forming of the inactive layer on the surface of the metal.Several organic inhibitors impeded the corrosion process by forming the invisible thin film on the metal surface.
e corrosion inhibition tendency of the synthesized 2amino-3,5-dicyano-4-aryl-6-substituted aminopyridine derivatives was tested by studying the weight loss of steel coupons immersed in a solution of 6 M HCl for nine daytime intervals of immersion at room temperature.e weight loss (gravimetric method) is known to be the most widely used method of monitoring inhibition efficiency.e results of weight loss of steel coupons with the addition of different concentrations 200, 400, and 800 ppm of different inhibitors during 1, 3, 5, 7, and 9 days of immersion in 6 M HCl are 2 Journal of Chemistry showed in Table 2 [28][29][30][31].e corrosion rate (k), the inhibition efficiencies (η w ), and the degree of surface coverage (θ) were calculated from the following equations: where S is the total area of the specimen, t is the immersion time, and W o and W are the values of the weight loss in the absence and/or presence of different concentrations of the inhibitors.Data in Table 2 and Figures 1-3 show that the synthesized 2-amino-3,5-dicyano-4-aryl-6-substituted aminopyridine derivatives 1, 5, 6, and 8 protected steel from corrosion.e weight loss decreases, and inhibition efficiency increases in the presence of inhibitors.As a result of weigh loss of mild steel in 6 M HCl with and without addition of various concentration of 2-amino-3,5-dicyano-4-aryl-6-substituted aminopyridine derivatives.
e efficiency increase with increasing the concentration of the inhibitor, which elucidated that the number of molecules adsorbed increased over the steel, blocking the active sites from acid and protecting the steel from corrosion.At 800 ppm which is the highest concentration, also, the best inhibition achievement of the derivative 5 was impute to presence of two methoxy groups attached to the phenyl.ese electron groups increase the resonance capability toward conjugations owing to the presence of unshared electron pairs on the

Experimental Section
All melting points measured on a Gallenkamp electric melting point apparatus are uncorrected.e infrared spectra were recorded in potassium bromide disks on a pye Unicam SP-3-300 and Shimdzu FT IR 8101 PC infrared spectrophotometers at the central laboratory of faculty of science, Ain Shams University.
e 1 H-NMR spectra were recorded on a Varian Mercury VX-300 MHz, using TMS as an internal standard in deuterated dimethylsulphoxide (DMSO-d 6 ).Chemical shifts are measured in ppm.e mass spectra were recorded on a Shimadzu GCMS-QP-1000EX mass spectrometer At 70 eV, elemental analyses were carried out at the microanalytical center of Ain Shams University.All the reactions and the purity of the new compounds were monitored by TLC using TLC aluminum sheets silica gel F 254 .

Experimental for Corrosion.
Coupons of steel were cut into 1 × 1 × 0.5 cm 3 dimensions are used for the gravimetric method.e specimens are washed, dried, and weighted.en, coupons were immersed in a beaker containing 50 ml of a solution of 6 M HCl for 9 days with different concentrations of the synthesized 2-amino-3,5-dicyano-4-aryl-6substituted aminopyridine derivatives.e specimens were washed, dried, and reweighted to take the difference in weight of steel coupons with and without the inhibitors, corrosion rate (CR), inhibition efficiencies (η (%)), and the degree of surface coverage (θ) for different concentrations at room temperature.

Conclusion
In summary, we have developed for the first time one-pot multicomponent reaction under the fusion condition without using solvent and catalysts.With this method, a wide range of novel 2-amino-3,5-dicyano-4-aryl-6-substituted aminopyridine derivatives were synthesized in high yields with a board substrate of functional groups.e synthesized pyridine derivatives act as corrosion inhibitors, and the rate of inhibition efficiency increases with the increasing concentration of the inhibitor.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Figure 2 :
Figure 2: Relation between inhibition efficiency of aminopyridine derivatives in 6 M HCl and time at room temperature at 400 ppm.

Figure 3 :
Figure 3: Relation between inhibition efficiency of aminopyridine derivatives in 6 M HCl and time at room temperature at 800 ppm.

Table 1 :
Optimization of reaction conditions.